In the past, a direction control valve was used which was operative to automatically direct the exhaust of a single acting function to the fluid reservoir and to direct the exhaust of continuously acting and/or double acting functions to a charging circuit for the supply pump as described in the U.S. Pat. No. 3,973,398 granted to C. E. Kittle on Aug. 10, 1976.
An accepted disadvantage of the prior art is that the exhaust of continuously acting and/or double acting functions must be at the same pressure as the charge circuit so that the exhaust fluid can provide the bulk of the fluid feeding the supply pump. Since the useable pressure for a function is the difference between its supply and exhaust pressures, it is evident that a power penalty exists in the form of the difference between reservoir and exhaust pressures. Where it has been desired to increase function power output without increasing the size of the function device itself, those skilled in the art have increased the supply pressure by providing a larger pump with the acceptance of this power penalty.